1. Field of the Invention
The present invention relates to a process for preparing an aromatic polyimide film having excellent thermal dimension stability from a solution of a polyimide precursor (for example, an aromatic polyamic acid) obtained from a biphenyl-tetracarboxylic acid and a phenylene diamine by a solution casting method or the like. More particularly, the present invention relates to a thermally stable aromatic polyimide film having a high thermal dimension stability (or a low thermal size change) and an average linear expansion coefficient which is substantially equal to the linear expansion coefficient of a ceramic or metal, and a process for the preparation thereof.
This polyimide film is very valuable as a base film of a printed circuit board ordinarily used for an electric or electronic part.
2. Description of the Related Art
An aromatic polyimide film obtained from pyromellitic dianhydride and an aromatic diamine is used as a typical instance of the aromatic polyimide film. Since the average linear expansion coefficient of this polyimide film in the unoriented state is as large as about 3.5.times.10.sup.-5 to about 4.5.times.10.sup.-5 cm/cm..degree. C., when the film is bonded to a copper foil or the like at a high temperature, a large amount of curling occurs.
As means for solving this problem, GB-A-No. 1098556 proposes a method in which the drawing operation is carried out at the film-forming step to reduce the average linear expansion coefficient of the film. However, the film obtained according to this method is defective in that the ratio of the change of the size at normal temperature after the heat treatment where the temperature is elevated to about 400.degree. C. from normal temperature and the film is maintained at this temperature for 2 hours (thermal dimension stability) is very bad, and therefore, the film cannot cope with increase of the wiring density in a wiring pattern.